This is a place to collect test ideas.

Proposal: dotnet/csharplang#2145

Specification

• Specification checked in to csharplang link
• Confirm codegen (use helper method from BCL, and/or fallback to generating if/throw)

Infrastructure

• LangVersion check
• IOperation/CFG nodes

Parsing

• string s !!= null parsed as string s!! = null

Generics

• void M<T>(T value!) { } is OK
• void M<T>(T value!) where T : struct { } is an error
• void M<T>(T value!) where T : unmanaged { } is an error
• void M<T>(T value!) where T : notnull { } is OK
• void M<T>(T value!) where T : class { } is OK
• void M<T>(T value!) where T : SomeStruct { } is an error
• void M<T>(T value!) where T : SomeClass { } is OK

Iterators

Null checking should always happen in initial call, not lowered methods

• Top-level iterator method returning IEnumerable
• Top-level iterator method returning IEnumerator
• Local function returning IEnumerable
• Local function return IEnumerator

Constructors

Null checking should be the first thing in a constructor

• Constructor with field initializers
• Constructor with this call
• Constructor with base call
• Record and record struct primary constructors

Nested functions

• Null-checked lambda parameter
• Null-checked local function parameter
• Null-checked lambda parameter inside field initializer
• allowed in anonymous method syntax delegate (object o!!) { }
• allowed in lambda parameter without explicit type (with and without parentheses) x!! => x, (x, y!!) => y
• allowed in lambda discard parameter _!! => { }

Warnings and errors

• Warn on null-checked parameter with default null value
• Warn on string? x! (null-check doesn't make sense on those)
• Error on int x! parameters
• Error on out string x! (null-check doesn't make sense on out parameter)
• Error on null-check on declarations without bodies (interface members, abstract, extern, declaration portion of a partial method)

Misc.

• uses reference equality (user-defined conversions are ignored)
• no boxing in check for S? s!! for struct type S
• check argument in getter and setter of indexer

Misc (from discussion in LDM)

• Null-checked params parameter (allowed, checks the array, not the elements)
• test void M<T>(T t!) where T : notnull { ... } (ok)
• should we use helper type from BCL to throw exception?
• Null-checked pointers and function pointers (works as expected)
• error on __arglist!!

From test plan review

• confirm CFG design with API review (delaying resolution till after feature merge) Parameter null-checking public API #58307
• LDM: allow on lambda parameter without type or parentheses? resolution: Allow.
• LDM: allow on lambda discard parameter? resolution: Disallow. (delaying till after feature merge) Disallow null checking discard parameters #58952
• async iterator methods
• LDM: allow on ref or in parameters? resolution: Allow. Allow 'ref' and 'in' parameters to be null-checked #58938
• check for unconstrained T does not box when T is a nullable value type (include a ThrowIfNull<T>(T value, string parameterName) helper method?) (delaying till after feature merge)
  • We don't box, but we also don't call the 'Throw' helper, which could improve inlining behavior of methods with null checked Nullable<T> parameters.
• disallow in delegate declaration
• override and interface implementation with/without !! is allowed
• SymbolDisplay is not affected
• IDE: classification, formatting, SyntaxNormalizer, quick info, typing, completion

Productivity

• Verify formatting in IDE
• verify that IDE doesn't gray out null-checked parameter which is otherwise unused
• need an analyzer/fixer to convert codebase (delaying till after feature merge) Parameter null-checking analyzer and fixer #58182
• Update F1 service a la Handle more cases in F1 help service #60156